This invention relates to electronic packages to house one or more microelectronic devices such as silicon based semiconductor chips. More particularly, the inner lead tips of a leadframe are supported to prevent deflection during wire bonding.
Electronic packages provide environmental protection for the delicate circuitry formed on a face of a semiconductor device. The packages also provide electrical interconnection between the circuitry and external circuits, typically through a leadframe or terminal pins. One electronic package has a metallic base and cover, the combination defining a cavity. A leadframe is disposed between the base and cover and adhesively bonded to both. A semiconductor device is supported on either a centrally positioned leadframe paddle or directly on the package base. Electrical interconnection between the device and the leadframe is by small diameter bond wires. This type of package is illustrated in U.S. Pat. Nos. 4,897,508 and 4,939,316, both by Mahulikar et al and both incorporated by reference in their entireties herein.
The leadframe is a thin, typically on the order of 0.15 mm to 0.25 mm (0.006-0.010 inch), metallic strip formed from copper or an iron-nickel alloy. The metallic strip is patterned into a plurality of leads with inner lead portions approaching a central aperture and outer lead portions extending outward from the central aperture for electrical interconnection to external circuitry.
In some leadframes, the central aperture contains a paddle formed from the same metallic strip as the leadframe and supported by outward running tie bars. The paddle facilitates assembly of the package. A semiconductor device is mounted to the paddle and small diameter bond wires electrically interconnect the device to the leadframe. The leadframe assembly is not yet housed within the electronic package and the inner leads are supported by external fixturing to prevent deflection during wire bonding.
Other leadframes do not include this centrally positioned paddle and the semiconductor device is bonded directly to the base of the electronic package. The inner lead portions are supported by a polymer adhesive. When this polymer adhesive is cured, nonuniform adhesive height can develop leading to a loss of lead coplanarity. Excess polymer resin can bleed on to the inner lead tips making wire bonding difficult. After the polymer cures, and wire bonding is commenced, the heat and pressure of bonding can deform the adhesive, deflecting the inner leads. Inner lead deflection leads to a poor wire bond or a short circuit.
In electronic package configurations lacking a die attach paddle, there remains a need for an improved electronic package base to prevent inner lead deflection during wire bonding.